Variable speed transmissions are known that utilize variable diameter pulleys interconnected by a flat belt so as to create a drive pulley and a driven pulley. See, for example, U.S. Pat. Nos. 4,024,772, 4,295,836 and 4,714,452. The variable diameter pulleys utilized in known variable speed mechanisms or transmissions employ parallel belt support members disposed on a common diameter coaxial with the rotational axis of the pulley and adapted for radial adjustment so as to vary the relative rotational speed ratio between the drive and driven pulleys. Various types of belt support members are employed, such as rollers or bars having opposite ends extending through radial slots formed in axially spaced rotatably driven primary plates or discs and into spiral grooves formed in pairs of secondary discs disposed outboard of the radial slotted discs. The outboard pairs of secondary discs are rotatable with the driven primary plates and are also caused to rotate relative to each other so as to effect radial movement of the belt support rollers or bars within the radial slots relative to the rotational axis of the pulley. See for example, U.S. Pat. No. 896,160. See also U.S. Pat. No. 4,591,351 which discloses a variable radius pulley employing pairs of laterally spaced pulley sheaves to support parallel belt drive elements. Each sheave comprises two juxtaposed circular discs having oppositely directed spiral grooves that intersect and receive the ends of parallel belt support drive elements. The discs are rotated relative to each other for varying the radial positions of the belt drive elements and thereby the radius of the pulley belt drive.
Variable diameter pulleys for use in transmission mechanisms are also known that employ fluid pressure to move pistons or sliders radially and thereby vary the diameter of a transmission belt support surface, as disclosed in U.S. Pat. No. 5,547,428. A significant drawback in the known variable diameter pulleys employed in variable speed transmissions or belt drive systems is that they are relatively complex and do not provide the precision incremental adjustment desired for variable speed belt transmissions.